This invention relates to welding and thermal spray apparatus including a wire electrode with a sheath that contains a core of a multiplex composite powder, and method of using the apparatus to deposit a low porosity coating that is adherent, with high levels of alloyed refractory elements, and is low in oxide content.
The problems of wear and corrosion of metallic surfaces, such as the inside of industrial boilers, may be ameliorated by a judicious application of a protective coating. Such coatings generally require chemical compatibility with the substrate, compatibility in thermal expansion coefficients, low porosity, and good adhesion. When the coating is applied, a coating process must be selected to be compatible with the substrate, its topography, surface curvature, and satisfy constraints imposed by line-of-sight limitations, while maintaining a desired thickness and avoiding thermal distortion of the coating and substrate. Additional detail is found in such sources as R. W. Smith, xe2x80x9cThermal Spray Technology-Equipment And Theoryxe2x80x9d, ASM International (1992) and T. J. Mursell and A. J. Sturgeon, xe2x80x9cThermal Spraying of Amorphous and Nanocrystalline Metallic Coatingsxe2x80x9d, World Center For Materials Joining Technology, Mar. 1999, which are incorporated herein by reference.
Thermal Spray Technology and Processes
Traditionally, thermal spray techniques have demonstrated their capability to meet many of these requirements. In such processes, molten particles impact onto a substrate surface to the form a coating.
Thermal spray coatings are surface layers that provide wear-, erosion-, abrasion-, and corrosion-resistant surfaces used in the manufacture of original equipment (e.g. automobile engines) and to repair and upgrade equipment that is in service (e.g. municipal intrastructure). Thermal spray technology lies within the welding field because of similarities in the electrical equipment and the feedstock materials used in thermal spray, welding, and brazing.
Thermal spraying processes deposit finally divided metallic or non-metallic feedstock materials (including wire and cored wires) that are heated to their molten state by, for example, an electric arc. The molten material is then accelerated toward the substrate by a high velocity fluid such as air. The particles or droplets strike the surface, flatten, and form thin platelets (splats) that conform, adhere, and interlock with surface irregularities of the substrate and with each other. As the sprayed particles impinge on the substrate, they cool and accumulate into a coating formed upon the surface.
The properties of a thermal spray coating depend on factors such as porosity, inter-particle cohesion, adhesion to the substrate, and chemistry of the coating material in relation to that of the substrate. Particles bond to the substrate metallurgically, chemically, and mechanically.
Thermal spray processes have relatively high coating rates due in part to their high-temperature heat sources. Additionally, such processes involve wide compositional ranges compared to other PVD or CVD coating processes. Coating materials formed thereby effectively serve as a new surface material added to the original surface (substrate).
Variants of thermal spray technology are capable of processing a wide spectrum of materials, thus making this technology an attractive choice of coating method. Metals from aluminum to tungsten, ceramics, polymers, and advanced composite materials are now being thermally sprayed. Interest in thermal spray processes and equipment has been stimulated by the versatility of the process, the demands of hostile operating environments and materials which need cohesive and adhesive coatings, and the development of new materials and surfacing concepts.
Thermal spray is limited, however, in that it is primarily a direct xe2x80x9cline-of-sightxe2x80x9d process where the incident droplets are deposited only onto surfaces that are aligned with the path followed by the incident droplets.
The Wire-Arc Spray Process
A wire-arc spray process is a type of thermal spray process that traditionally utilizes a DC electric arc to directly melt consumable electrode wires. In twin wire-arc spraying, the electric current is carried by two electrically conductive, consumable wires. An electric arc is created between the wire tips across a gap created by the continuous convergence of two wires. A gas jet blows from behind the converging electrodes and transports the molten metal that continuously forms as the wires are melted by the electric arc. High-velocity gas breaks up or atomizes the molten metal into smaller particles in order to create a fine distribution of molten metal droplets. The droplet-infused gas then accelerates the particles away from the electrode tips to a substrate surface, where the molten particles impact to incrementally form a coating upon cooling. Unlike combustion or plasma spraying, the droplets are already molten when they enter the jet zone, and are continuously cooled in transit to the substrate as the particles leave the arc zone. Thus, the optimized wire-arc spray process attempts to shorten the time in transit so that the cooling effects on the molten droplets do not reduce the amount of molten particles that are needed to form continuous coating layers.
Particles created by most conventional wire-arc spray processes tend to be larger and more irregularly sized than those found in powder-fed thermal spray coating processes. This size irregularity contributes to unwanted deposit porosity, which is common in many wire-arc spray coatings. Droplet atomizing irregularity also depends on uniformity in wire feed rates, the stability of electrical voltage and current, and nonuniformity in the arc gap. The attainment of high-density wire arc coatings is to some extent possible with careful control of wire feed rates, the use of smaller diameter wires, the application of lower currents and wire feed rates, and by experimenting with the atomizing gas-to-metal feed rate.
The consumable two wire-arc technique is responsible for the growth of metallizing as a commercial coating method, especially using aluminum and zinc alloys. Smith, supra, p. 23. Despite the wire arc""s high deposit rate capability, it is limited to materials that are conductive and to those materials which are ductile enough to be formed into wire. This reduces the number of materials that are suitable for use in wire-arc processes. However, recent cored wire development has expanded the range of materials to include cermets and xe2x80x9chardfacingxe2x80x9d carbide/oxide with metal matrices. Id. However, such advances still leave the unsolved problems of adhesion to the substrate and cohesion within an often porous coating.
The Cored Wire Electrode
Mechanical, e.g., folded, abutted, or lapped seams of the tube or sheath that contains a core powder found in conventional electrodes present problems when used in welding and thermal spray applications.
The longitudinal seams of sheaths formed in conventional processes are closed by the compression stress, which results from reduction operations. Once the forming and sizing are complete, these cored wire electrodes are traditionally wound on spools for feeding into the wire or welding apparatus, or into coils or some other packaging, including drums that contain the wire for stowage, shipping and application. This winding process may relieve the mechanical compression stress that provides the seam""s integrity, causing the seam to open and release or spill the core contents. If the wire has low ductility, it may break. When the wire is being used for welding or thermal spraying, the apparatus includes a wire feed drive system, conduit/liners or guide tubes, welding nozzle/thermal spray front end and a power source. The wire is fed from its container through a set of drive rolls that apply a pinching effect to the wire, which can cause the seam to open. The wire also passes through a flexible conduit that protects and guides it from the spool or package payoffs to the welding or thermal spray gun (head). Any movement of the gun or flexible conduit will also bend the wire, and may cause the seam to open or brittle wire to break.
Open seams reduce the strength of the wire and allow the contents to spill. This tends to clog the conduit and reduce the effectiveness and performance of the torch/gun, create amperage drops, cause unpredictable feed and spray rates, allow binding in the contact tips and complete stoppage of the application.
Additionally, inconsistent seam integrity will create problems with the transfer of electrical current across the anode-cathode gap. Electrical current primarily flows around the surface of the wire. In welding and thermal spray processes, the wire is energized at a point as close as possible to the arc to create the least electrical resistance, consistent current flow and to reduce heating of the wire. The current is transferred to or from the wire through electrical contact tips. The contact tips, which are normally copper or brass, have a very close tolerance and limited contact surface. Any interruption in current flow caused by an irregular shape of the wire will be manifest in complications with the weld deposit or thermal sprayed coating.
Further, sealed seams, when their integrity is weakened, may permit moisture to penetrate past the sheath/tube and become absorbed in the core material, which causes porosity in coatings, and porosity and hydrogen embrittlement in welds.
Deposit Characteristics
Coatings deposited by wire arc spray techniques generally contain pockets of metal that are thicker and more varied in size than those found in plasma spray coatings deposited by powder combustion and related conventional processes. Large areas of porosity may result from solid, larger particles being trapped, thereby creating voids that have not been filled by subsequent droplets. The conventional wire-arc spray deposit""s coarser microstructure and porosity are often attributed to the large and irregular droplet sizes caused by wire-atomization irregularities. Wire arc spray coatings tend to be rougher than those formed by other thermal spray processes, especially where large diameter wires are used.
Many municipal and civil structures such as bridges, water storage tanks and marine coatings for ship stacks and decks are coated with wire arc spray apparatus.
Against this background, there remains a need to eliminate or effectively reduce surface problems associated with high levels of oxidation, corrosion, and abrasion in industrial environments, including, but not limited to the power generation boiler environment, the paper and pulp industry digester, petro-chemical and chemical manufacturing processing equipment, turbo machinery (jet and steam turbine) and industrial combustion engines.
These and related challenges have led to the development by the inventors of a thermal spray apparatus that includes at least one cored wire electrode with a sheath. The sheath contains a multiplex powder composite selected from a group of particle sizes consisting of micron (1xc3x9710xe2x88x926 meters) and sub-micron (less than 1xc3x9710xe2x88x926 meters), including nano-scale (1xc3x9710xe2x88x929 meters) particles for a core material in a sheath that serves as an electrode in a wire arc spraying or welding apparatus.
The cored wire electrode manufacturing process involves taking a flat metallic strip material, forming it into a xe2x80x9cUxe2x80x9d shape, filling it with the granular or powder core material, then roll forming it into a tube and reduction drawing or rolling it to a suitable size for welding or thermal spraying applications. See, e.g., commonly owned U.S. Pat. No. 5,479,690 which is incorporated herein by reference.
The process for manufacturing a sheath or welded seam product requires changes in conventional procedures in order to address issues that are inadequately addressed by traditional methods. The first problem is how to continuously form a tube and weld the seam. The second challenge is how to get particles into the sheath during the forming and seam welding operation without affecting the quality of the weld or the quality of the core material. Good results have been obtained in manufacturing by orienting the sheath in a vertical position using forming dies, guide rollers, pinch rolls for welding and a fill tube that allow deposit of the core material at a point beyond the welding operation. See, e.g., commonly owned U.S. Pat. Nos. 5,346,116; 5,328,086; 5,479,690, which are incorporated herein by reference.
The seamless tube or sheath and the composite particles when sprayed provide a more dense coating than is currently available with conventional wire arc thermal spray materials. The added efficiencies, the ability to gain greater reductions, and core compaction enable a cored wire to be made that contains less than 10% voids by volume and a relatively uniform particle distribution. Additionally, chemistries can be balanced based on the sheath and core material, which enhance the versatility and effectiveness of the wire in producing the desired coating or weld.